High energy picosecond-class lasers at wavelengths where energy storage materials don't exist commonly rely on optical parametric amplification (OPA). Parametric gain is an instantaneous process, it exists only when the pump pulse is present. This requires that the signal pulse to be amplified is well synchronized with the pump pulse. Any temporal jitter between the signal and pump is translated into amplitude and spectral shifts (jitter). One design option to ease the difficulty of temporal synchronization is to stretch the signal pulse to the nanosecond regime and use a nanosecond class pump laser.
Pulse stretchers and compressors, however, come with their own particular difficulties. Traditional pulse stretchers and compressors are very large devices having many small components with alignment tolerances on the scale of sub-millimeters. A small pulse stretcher may require up to 8 cubic feet of volume. A pulse stretcher suitable for high-energy, picosecond-class pulses would be even larger. Furthermore, because tolerances for a pulse stretcher are comparable to those for an interferometer, and because alignment requirements are on a scale of sub-millimeters throughout the device, a pulse stretcher is a large, cumbersome, expensive device that is highly sensitive to vibration, dust, and any other potential source of disturbance. It is difficult to configure and keep stable in a lab setting and utterly unfit for use in any kind of field or mobile environment.
A typical pulse compressor is similar to a typical pulse stretcher in terms of overall volume, components, tolerances, alignment requirements, high cost, low efficiency, and general unsuitability for use in anything other than a lab setting.
The gain coefficient of an optical parametric amplifier (OPA) is directly related to the pump pulse intensity. Using short pump pulses has the advantage of increased intensity as well as increasing the mixing crystal damage threshold (damage threshold flux increases approximately as the square root of the pulse width). It is therefore desirable to use a picosecond class laser as the pump. But, as noted previously, this makes the temporal synchronization an extremely difficult task because at such pulsewidths the system jitter is much larger than the width of a single laser pulse.
Past work on using nanosecond lasers to pump OPAs relied on electronically synchronizing the nanosecond laser to a master oscillator either optically or electronically. Precision electronic delay boxes were used to over-lap the signal and pump pulses inside the mixing crystals. This technique can be used when the laser pulses are in the multiple nanosecond time regime. A seeded nanosecond laser itself has a timing jitter on the order of a nanosecond. However, for a picosecond-class laser, the timing jitter will be on the order of 100 or so ps or less. At such tolerances and durations, electronic delay boxes and similar devices are simply not fast enough to be effective.
Furthermore, with mobile or field applications in mind, pulse stretchers and pulse compressors should preferably be omitted as they do not contribute to the reliability or stability of a laser system. It would therefore be an advance in the art to create a laser system capable of being synchronized without the use of pulse stretching/compression or electronic delay components.